Continuous casting of steel



United States Patent 3,411,564 CONTINUOUS CASTING OF STEEL Eldon D.Miller, Jr., Bridge ville, Pa., assignor to Dresser Industries, Inc.,Dallas, Tex., a corporation of Delaware No Drawing. Continuation-impartof application Ser. No.

615,077, Feb. 10, 1967. This application May 17, 1967,

Ser. No. 639,013

3 Claims. (Cl. 164-73) ABSTRACT OF THE DISCLOSURE A high-temperatureliquid-solid lubricant suitable for using in the continuous casting ofmolten steel comprising submicron spheroidal oxides not susceptible tophase changes at temperatures up to 1600 F. and a carrier having a highflash point and a low carbon residue on burning.

Related case This case is a continuation-in-part of application Ser. No.615,077, filed Feb. 10, 1967 having the same title, now abandoned.

Background The commercial use of processes for the continuous casting ofsteels seems destined to take an increasingly important position infuture steelmaking. Its many advantages in terms of cost, labor, andsimplicity make it very attractive to an industry becoming highlyautomated. The concept of continuous casting of steel is not new,however. As early :as 1865, Sir Henry Bessemer envisioned the continuouscasting of a steel plate or the like by continuously pouring moltenmetal between two water-cooled rollers to recover a strip of steel.

A recent article 1 is recommended for a detailed study and analysis ofthe evolution of processes for the continuous casting of steel from theearly work of Bes semer to many contemporary processes. As pointed outby the article, two of the more important and critical parts or steps incontemporary processes for the continuous casting of steel areconsidered to be:

(1) The actual pouring of a carefully regulated stream of molten metalinto an initial freezing stage or mold, and

(2) The actual cooling or freezing step itself.

In this latter step, a peripheral skin or shell is formed about a coreof molten metal as it passes through the cooling stage or mold. Thisskin or shell formation must be carefully controlled if a breakout ofmolten metal is to be prevented. The eventual recovery of a billet offairly uniform dimensions also depends on the skin formation. If thestream of molten metal being poured into the cooling or freezing stageis not constant in volume and cross-sectional configuration, theformation of a uniform and sufliciently strong skin is made diflicult,if not impossible.

In many of the present practices, the semisolidified stream is subjectedto a pulling action by frictional engagement with driven rollers, or byfrictional engagement with upwardlyand downwardly-reciprocating moldwalls in the initialcooling stage. Of course, gravity itself can be ofsubstantial effect in placing stresses on the evolving semisolidifiedstream because of the substantially vertical process flow path presentpractices utilize.

One of the more essential factors controlling the freezing step islubrication of the mold. The molds are typically water-cooled copperwith highly machined surfaces. Their temperature never exceeds about2000 F., which Leonard V. Gallagher et a1., authors, ScientificAmerican, December 1963.

is about the melting point of copper. However, portions of the mold havea temperature exceeding 1000 F. Obviously, ordinary oils and greaseswhich begin to oxidize badly at temperatures above 600 F. are notadequate. Rapeseed oil is the traditional lubricant now being used bycontinuous casting operators. Several synthetic oils, such assilicone-based oils, have been used with limited success. One unusuallubricant used is a mixture of fly ash :and alkali oxides, which createsan extremely fluid slag. Typical solid lubricants, such as graphite, andthose of the molybdenum disulfide type, cannot be used as they wouldbuild up on the copper mold and tend to contaminate the steel surface.

The general requirements of a lubricant for continuous casting are thatit prevents welding of the liquid steel to the solid copper mold wall,and that it reduces the sliding friction between the skin and the newlyformed casting and the mold wall. More specific requirements of fluidlubricants are that they must not break down at temperatures under 400F., they must not produce coke deposits, and they must not contaminatethe casting surface.

It is an object of this invention to provide an improved lubricant forthe continuous casting of steel which has all of the good qualities ofrapeseed oil, but which has improved high-temperature lubricatingcharacteristics.

Brief description of the invention Broadly, according to this invention,there is provided a method of continuously casting molten steel andwatercooled copper molds. According to this invention, the molds arelubricated with a composite liquid-solid lubricant comprising from 10 topreferably 10 to 60%, by weight, submicron spheroidal oxides notsusceptible to phase changes at temperatures up to 1600 F. and 20 topreferably 40 to 90%, by weight, of a carrier having a high flash pointand a low carbon residue on burning. Suitable submicron spheroidal oxideis volatilized silica which is manufactured by the oxidation andcollection of silicon vapors. Submicron spheroids of titania and zincoxide are also available and suitable. Suitable carriers includerapeseed oil, crambe oil, glycerol, polyethylene glycol, andsilicone-based oils.

In this specification by high flash point I mean in excess of 300 F. Bylow carbon residue I mean equivalent to rapeseed or crambe oil. By phasechange I mean crystallographic rearrangement.

Detailed description of invention This invention is a considerabledeparture from the modern theories of lubrication. Lubricants areusually considered to be fluids which, through the phenomena ofhydrodynamic pressure, separate two moving surfaces. Solid lubricantsare typically materials such as graphite or molybdenum disulfide whicheasily shear or flow similar to fluids. Applicants lubricant is acombination of a fluid lubricant and a nonshearing solid. The solidportion of applicants lubricant is comprised of submicron spheroidaloxides which behave similar to thousands of roller bearings. Thespheroidal particles have a tendency to increase the thickness of thecarrier oil film, thus improving the fiuid lubrication so long as thecarrier fluid lasts. Thereafter, the spheroidal oxides must act as a drybearing. The oxides are about twenty times more effective than oil filmsfor conducting heat away from the steel billet and, hence, improve theefliciency of the copper molds.

The submicron spheroidal oxide must not experience a phase change belowabout 1600 F. This is because phase changes are usually accompanied bychange in volume or shape and other properties such as strength. Oxidesare used because they have little tendency to react with the surface ofthe billet as would, for example,

metals. I have found that a particularly suitable oxide is vitreoussilica. Submicron spheroids of vitreous silica can be prepared by vapordeposition and are commercially available. Submicron spheres of titaniaand zinc oxide are also commercially available and suitable according tothis invention.

Carrier fluids used according to this invention should have a flashpoint in excess of 300 F., and should burn with no metallic orcarbonaceous residue. Carrier fluids should have a viscosity suflicientto hold the spheroidal particles in suspension at room temperatures. Lowviscosity liquids such as kerosene fail to hold volatilized silica insuspension.

The following are examples of liquid-solid lubricants according to theteachings of this invention A mixture of 23% volatilized silica and 77%glycerol was combined and agitated until the volatilized silica wascompletely dispersed within the glycerol. This is the best mode nowknown for the practice of this invention. The lubricant was submitted totwo tests to determine its tendency to ignite. The lubricant was firstspread over a concrete surface and subjected to the flame of a blowtorch. It did burn but there was no explosion, and no organic residueremained. In another test, the lubricant was poured on a brass platewhich was water-cooled on its other face. The lubricant was subjected tothe flame of a blow torch but could not be ignited.

Another lubricant according to this invention was prepared by mixing 60%volatilized silica and 40% ethylene glycol. In certain continuouscasting machines operating at higher pressures as much as 80%volatilized silica or other submicron spheroidal solid is desirable inthe lubricant.

The lubricant film is provided in continuous casting molds through slitsin the mold wall, about 0.020 inch high, and having a width to suit. Thelubricant runs down the side of the mold wall from the slits. Thelubricant is supplied to the slits from a well inside the mold wallwhich is in turn connected to the lubricants source. The lubricant isusually delivered by a pump which has an output of about 0.4 to 8.0gallons per hour. The lubricant usage is in the range of 0.7 to 1.0pound per ton of steel.

Having thus described the invention in detail and with sufficientparticularity as to enable those skilled in the art to practice it, whatis desired to have protected by Letters Patent is set forth in thefollowing claims:

I claim:

1. In a method of continuously casting molten steel by introducing amolten stream of steel into water-cooled copper molds and freezing thestream within the molds, the improvement comprising the steps of:

(1) lubricating a portion of the mold walls by introducing a compositeliquid-solid lubricant comprising from 10 to by weight, submicronspheroidal oxides selected from the group consisting of titania andsilica and 20 to by weight, of a carrier fluid having a high flash pointand a low carbon residue on burning, which lubricant provideshydrodynamic lubrication, and

(2) lubricating the remaining portion of the mold walls with thesubmicron spheroidal oxides of the lubricant introduced in step 1 afterthe carrier fluid has been burned away, which lubricates in a mannersimilar to a plurality of dry bearings.

2. The method of claim 1 in which the carrier fluid is selected from thegroup consisting of rapeseed oil, crambe oil, glycerol, polyethyleneglycol, and silicone-based oils.

3. The method of claim 1 in which the lubricant comprises 10 to 60%, byweight, submicron spheroidal oxides.

References Cited UNITED STATES PATENTS 2,419,252 4/1947 Bychinsky et al.2s2 25 2,466,642 4/1949 Larsen 252-2s 2,609,342 9/1952 White et al252-25 2,926,138 2/1960 Huet 2s2 25 3,103,491 9/1963 Wright 2s2-2sFOREIGN PATENTS 700,512 12/1964 Canada. 1,138,493 10/1962 Germany.

DANIEL E. WYMAN, Primary Examiner.

I. VAUGHN, Assistant Examiner.

